1. Field of the Invention
The present invention relates to photovoltaic elements such as solar cells and sensors, particularly to a photovoltaic element comprising an nip-type silicon semiconductor layer containing an i-type microcrystalline semiconductor layer.
2. Related Background Art
Photovoltaic elements, which are photoelectric conversion elements for converting solar insolation to electric energy, are used widely as a small power source for consumer goods such as electronic calculators, wrist watches, and the like, and are considered to be promising as an alternative power source to chemical fuels such as petroleum and coal.
The photovoltaic element utilizes photoelectromotive force of a pn junction of a semiconductor. The semiconductor such as silicon is allowed to absorb sunlight to produce photocarriers of electrons and holes, and the photocarriers are drifted by an internal electric field of the pn junction and are unmoved. A photovoltaic element can be produced in a in a manner similar to an usual semiconductor manufacturing process. Specifically, a p-type or n-type valency-controlled single crystal silicon is made by a crystal growth process such as the CZ technique, and the single crystal is sliced into silicon wafers of about 300 .mu.m thick. Further, on a surface of the wafer, a layer of a conductivity type other than that of the wafer is formed by an appropriate means such as diffusion of a valency controlling agent to produce a pn junction.
Incidentally, from the standpoint of reliability and conversion efficiency, single crystal silicon is now used mainly for practical photovoltaic elements. However, the production cost thereof is high since the semiconductor process is employed for the production. The single crystal silicon photovoltaic element has further problems in that the single crystal silicon is an indirect transition semiconductor and has a small optical absorption coefficient, whereby at least a thickness of 50 .mu.m of the single crystal photovoltaic element is necessary for absorption of incident sunlight, and that the single crystal silicon has a bandgap of about 1.1 eV which is narrower than the value of 1.5 eV suitable for the photovoltaic element, whereby the short wavelength components of the light can not be utilized effectively.
On the other hand, a photovoltaic element employing polycrystal silicon is disclosed which has a surface texture structure, formed at a process temperature lower than 550.degree. C., and exhibits excellent characteristics with a thickness of 5 .mu.m or less (Keiji Yamomoto, Akihiko Nakajima, et al., "Optical Confinement Effect for below 5 .mu.m Thin Film Poly-Si Solar Cell on Glass Substrate", Jpn. J. Appl. Phys., Vol. 36 (1997), pp. L569-L572). However this photovoltaic element has not been practically used due to a relatively high process temperature and a low process speed.
Further, since the single crystal and the polycrystal materials are crystalline, it is very difficult to produce a wafer or polycrystal layer with a large area. Moreover, when the photovoltaic element employing such a material is used outdoors, an expensive mounting equipment is required in order to protect the element from mechanical damage caused by various weather conditions. Therefore, there is a problem in that the production cost for unit power generation is high in comparison with conventional power generation methods.
Therefore, reducing cost and expanding the area are an important technical subject for practical use of the photovoltaic elements in power generation. Many investigations have been made, and low-cost materials and materials of higher conversion efficiency have been searched. Such materials for the photovoltaic element include amorphous semiconductors of the tetrahedral type such as amorphous silicon, amorphous silicon germanium, and amorphous silicon carbide; compound semiconductors of Groups II-VI such as CdS, Cu.sub.2 S, etc. and those of Groups III-V such as GaAs, GaAlAs, etc., or the like. Especially, thin film photovoltaic elements employing an amorphous semiconductor as the photoelectromotive force generation layer are considered to be promising because of their advantages over the single crystal photovoltaic elements such as a possibility of producing films of a larger area, attainability of a smaller film thickness, ability to be deposited on any substrate material, and so on.
However, when the photovoltaic element employing the amorphous semiconductor is to be used as a power supply element, there still remains a problem that photoelectric conversion efficiency and reliability need to be further improved.
As the means for improving the photoelectric conversion efficiency of the photovoltaic element employing the amorphous semiconductor, there is carried out, for example, a process of narrowing the bandgap to increase the sensitivity to the longer wavelength light. Specifically, since the amorphous silicon is not capable of absorbing and effectively utilizing longer wavelength light of 700 nm or more due to its bandgap of about 1.7 eV, materials which have a narrower bandgap and are sensitive to the longer wavelength light. As such a material, there is included amorphous silicon germanium the bandgap of which can easily be varied arbitrarily in the range from 1.3 eV to 1.7 eV by changing the ratio of the silicon source gas to the germanium source gas during the film formation.
Further, as another method for improving the photoelectric conversion efficiency of the photovoltaic element, U.S. Pat. No. 2,949,498 discloses employing a stack cell in which a plurality of photovoltaic elements of the unit element structure are stacked. The stack cell employs a pn junction semiconductor. The concept is common to amorphous materials and crystalline materials and is to improve the power generation efficiency by efficiently absorbing the sunlight spectrum by photovoltaic elements of different bandgaps to increase V.sub.oc.
The stack cell is to improve the conversion efficiency by stacking elements of different bandgaps and thus efficiently absorbing respective fractions of the sunbeam spectrum, and is designed such that the bandgap of the so-called bottom layer positioned at the side opposite to the light incidence side of the stack cell is narrower than the bandgap of the so-called top layer positioned at the light incidence side and over the bottom layer. This enabled it to sufficiently absorb the spectra of the sunbeam and improve the photoelectric conversion efficiency (K. Miyachi et al., Proc. 11th. E. C. Photovoltaic Solar Energy Conf. Montreux, Switzerland, 88, 1992; and K. Nomoto et al., "a-Si Alloy Three-Stacked Solar Cells with High Stabilized-Efficiency, 27th Photovoltaic Science and Engineering Conf., Nagoya, 275, 1993).
However, the photovoltaic element employing the amorphous semiconductor for all the i-type semiconductor layers has encountered a problem of lowering the conversion efficiency by light irradiation, namely the so-called photodeterioration, and the reduction thereof has been limited. This is because the film of the amorphous silicon or the amorphous silicon germanium is deteriorated by light irradiation to lower the movability of the carriers. Such phenomenon is not observed in crystal semiconductor and is inherent to the amorphous semiconductor. Therefore, the amorphous semiconductor is less reliable and is not suitable for practical use for power generation.
Recently, attention has been focused on i-type microcrystalline silicon semiconductors produced by the plasma CVD process in view of smaller film thickness necessary for obtaining a sufficient photocurrent and of less photodeterioration and several reports have been presented thereon: A. Shah, H. Keppner et al., "INTRINSIC MICROCRYSTALLINE SILICON (.mu.c-Si:H), A PROMISING NEW THIN FILM SOLAR CELL MATERIAL", 1994 IEEE First WCPEC, pp.409-412, Dec. 5-9, 1994, Hawaii; and A. Shah, H. Keppner et al., "The "Micromorph" Solar Cell, Extending a-Si:H Technology Towards Thin Film Crystalline Silicon, 25th IEEE PV Specialists Conference, Washington, May 13-17, 1996.
However, in any of the above reports, the deposition rate of the microcrystalline layer is small, which is an obstacle to practical use.